createElement should not transmit exceptions but report them.

third_party/WebKit/LayoutTests/custom-elements/spec/resources/custom-elements-helpers.js

 function create_window_in_test(t, srcdoc) {
   let p = new Promise((resolve) => {
     let f = document.createElement('iframe');
     f.srcdoc = srcdoc ? srcdoc : '';
     f.onload = (event) => {
       let w = f.contentWindow;
       t.add_cleanup(() => f.parentNode && f.remove());
       resolve(w);
     };
     document.body.appendChild(f);
@@ skipping 30 matching lines @@
     assert_equals(actual[i][0], expected[i][0], callback + ' callback should be invoked');
     assert_equals(actual[i][1], expected[i][1], callback + ' should be invoked on the element ' + expected[i][1]);
     assert_array_equals(actual[i][2], expected[i][2], callback + ' should be invoked with the arguments ' + expected[i][2]);
   }
 }
 
 function assert_is_upgraded(element, className, description) {
   assert_true(element.matches(':defined'), description);
   assert_equals(Object.getPrototypeOf(element), className.prototype, description);
 }
+
+// Asserts that func synchronously invokes the error event handler in w
+// with the expected error.
+function assert_reports(w, expected_error, func, description) {
+  let old_onerror = w.onerror;
+  let errors = [];
+  w.onerror = (event, source, line_number, column_number, error) => {
+    errors.push(error);
+    return true; // the error is handled
+  };
+  try {
+    func();
+  } catch (e) {
+    assert_unreached(`should report, not throw, an exception: ${e}`);
+  } finally {
+    w.onerror = old_onerror;
+  }
+  assert_equals(1, errors.length);
+  // assert_throws has an error expectation matcher that can only be
+  // accessed by throwing the error
+  assert_throws(expected_error, () => { throw errors[0]; });
+}